1. Field of the Invention
The present invention relates generally to induction welding devices for welding metal tubes and more specifically to an impeder having an exposed ferrite core.
2. Description of the Prior Art
It is well known to those skilled in the art to use high frequency alternating current to weld the edges of a metal tube as it is being formed. Such seam welded metal tubing is generally formed by an apparatus, usually referred to as a tube mill. A flat metal strip, known as a skelp, is formed into a tube or tubing by bringing the edges of the skelp together and then seam welding the edges to form the tubing. As the metal skelp is moving longitudinally through the tube mill, the edges gradually converge at a point, generally referred to as the welding point, and an R-F alternating current is caused to flow along the converging edges and across the welding point. In this manner, the converging edges are preheated and then raised to the welding temperature at the welding point, where they are welded together. The current flow along the converging edges and across the welding point is caused by an inductor spaced above the edges and the welding point and energized by a high frequency power source. In such an apparatus, it has become common practice to provide an impeder within the tube or tubing and directly below the inductor, converging edges and the welding point. The impeder is typically formed from a high magnetic permeability material, such as a ferrite, and causes the current to be concentrated along the edges and across the welding point. Typically, the impeder is coupled to a source of mill coolant which flows over the ferrite to cool it during the welding process.
A representative example of such an impeder is disclosed in U.S. Pat. No. 2,833,910, issued May 6, 1958 and entitled "Tube Welding". The aforementioned patent discloses an impeder having a magnetic core material disposed within a copper sleeve. The magnetic core is prevented from sliding out of the sleeve by a pin located at the end of the sleeve. A cooling fluid or coolant is forced to flow between the sleeve and the magnetic core.
Another representative example of a prior art impeder is disclosed in U.S. Pat. No. 3,004,134, issued Oct. 10, 1961, and entitled "Impeder for Use on a Thermatool Welder". The aforementioned patent discloses an impeder having a bonded iron powder core formed around a bronze rod which are both disposed in fiberglass sleeve. The fiberglass sleeve is coupled to a source of coolant. The bronze rod and bonded iron powder core extend beyond the downstream end of the fiberglass sleeve, and the bonded powdered iron core is retained within the fiberglass sleeve by a cylindrical sleeve rigidly pinned to the bronze rod. A bead trimmer is connected to the end of the bronze rod.
The ferrite materials typically used to make the cores of impeders are very hard, brittle, almost glasslike materials which are extremely difficult to machine. In order to make an inexpensive impeder, simple rods of ferrite, which are not machined have been used as disclosed in the '910 patent described above. Such devices merely prevent the fully enclosed rod from sliding out of the sleeve by using a pin to block the end of the sleeve.
The end of the sleeve enclosing the end of the impeder tends to be damaged by molten metal causing the core to slide out, thereby requiring the replacement of the impeder. The replacement of the impeder is time consuming, since the tube mill must be shut down during the replacement process. Even though the impeder itself is relatively inexpensive, the down time of tube mill associated with the replacement of the impeder is an expensive and undesirable undertaking.
The impeder disclosed in the aforementioned '134 patent eliminates some of the problems associated with the impeder of the '910 patent. The impeder of the '134 patent, however, is formed from bonded iron powder telescoped over a bronze rod and is relatively expensive to manufacture. Moreover, due to the bead trimmer and the cylindrical sleeve pinned to the bronze rod it is impossibele to place the bonded powdered iron core in close proximity to the weld point. The close placement of the ferrite core to the weld point improves the efficiency of the welding process, but such close placement is not possible with the aforementioned prior art impeders. Accordingly, there is a need for a relatively inexpensive, long lasting impeder that can be placed in close proximity to the weld during the induction welding of metal tubing.